Abstract
Two types of chemical-pressure effects on the Verwey-type transition in Ba Sm Fe 2 O 5 are studied, viz., ionic-size contractions at the Ba and Sm sites. Whereas the replacement of Sm by the smaller Eu and Gd atoms increases the transition temperature TV from 232 K to 264 K , the Sr-for-Ba substitution decreases it from 232 K to 180 K for x=0.15 in (Ba 1 − x Sr x ) Sm Fe 2 O 5 . Of various structural parameters determined by synchrotron x-ray diffraction above TV, the magnitude of the orthorhombic distortion (=b− a ) is found to correlate with TV in a manner that unifies both substitution schemes. It is suggested that when an increase in b− a above TV is achieved by means of the structural substitution, it makes it easier for the dxz Fe 2 + orbital ordering to occur below TV.
© 2004 American Physical Society